Electromagnetic interference (EMI) is the generation of undesired electrical signals, or noise, in electronic system circuitry due to the unintentional coupling of impinging electromagnetic field energy. Any propagating electrical signal is comprised of an electric field (E-) and magnetic field (H-) component. The sinusoidal nature of the signal results in the tendency of circuit components, such as wires, printed circuit board conductors, connector elements, connector pins, cables, and the like, to radiate a portion of the spectral energy comprising the propagating signal. Circuit elements are effective in radiating spectral components which have wavelengths similar to the radiating element dimensions. Long circuit elements will be more effective in radiating low frequency noise, and short circuit elements will be more effective in radiating high frequency noise. These circuit elements behave just like antennae which are designed for the transmission of the radiating wavelengths.
Integrated circuits (ICs) which have output drivers that create pulses with high amounts of spectral energy are more likely than low power drivers to cause EMI due to the probable mismatch between the driver and line impedance, and the resistance to instantaneous signal propagation imposed by the parasitics of the conductor. For instance, CMOS (complementary metal oxide semiconductor) ICs which switch 5 volts in rapid rise times can have a large content of high frequency components and high spectral energy during operation. If the rise time of a propagating signal is less than the round trip propagation delay from source to load, then the conducting medium will behave as a transmission line. In such a connection, variations in the characteristic impedance [Z.sub.o, where: Z.sub.o .about.(L/C).sup.1/2 ; L=conductor inductance; C=conductor capacitance] along the line will cause perturbations in the electromagnetic field associated with the propagating signal. These disturbances in the electromagnetic field result in reflections of portions of the signal energy at the points where the variation occurred. If the signal is not totally absorbed by the load at the end of the conductor length, due to unmatched impedances or lack of proper line termination, the unabsorbed energy will be reflected back towards the source. These reflections give rise to radiated emissions. Proper termination and controlled impedance interconnections will reduce radiated noise significantly.
The coupling of signal energy from an active signal net onto another signal net is referred to as crosstalk. Crosstalk is within-system EMI, as opposed to EMI from a distant source. Crosstalk is proportional to the length of the net parallelism and the characteristic impedance level, and inversely proportional to the spacing between signal nets. Proper interconnect layout design can reduce the incidence of crosstalk. Strong sources of low impedance, H-field rich EMI are relatively high current and relatively low voltage components such as power supply, solenoids, transformers, and motors. If the H-field possesses high intensity, the field can induce spurious current flow in other system components. Thus, noise radiated from within a system can interfere with system performance by coupling with other system elements, not just adjacent conductor nets, as another form of within-system EMI.
Electronic systems are becoming smaller, and the density of electrical components in these systems is increasing. As a result, the dimensions of the average circuit element is decreasing, favoring the radiation of higher and higher frequency signals. At the same time, the operating frequency of these electrical systems is increasing, further favoring the incidence of high frequency EMI. EMI can come from electrical systems distant from a sensitive receiving circuit, or the source of the noise can come from a circuit within the same system (crosstalk or near source radiated emission coupling). The additive effect of all these sources of noise is to degrade the performance, or to induce errors in sensitive systems. The prevalence of high frequency systems and portable electronics is creating a very complex spectral environment for the operation of sensitive electrical systems. The Federal Communications Commission (FCC) and the Federal Aviation Association (FAA) regulations on radiated emissions are becoming increasingly difficult to meet without adding to system size, mass, or cost due to the need for the EMI shielding.
EMI shielding has taken many forms. Sensitive or radiating devices are often covered with a lid or enclosure which is connected to ground potential in the process of securing the cover in place. Shielding close to the source, where the field intensity is the highest, requires greater shield efficiency to contain the field. Therefore, in many cases it has been more common to shield the sensitive, EMI receiving component. In some instances, entire circuit boards are covered with a grounded lid. Polymer thick film conductor materials, such as a screen-printable copper filled epoxy paste, are sometimes used to form a shield. In other known EMI shielding methods, individual ferrite components are often placed on device pins or in series with a circuit to attenuate unwanted noise which may be causing system errors, or acting as sources of radiated emissions. In another application of ferrite beads or elements, a ferrite component is used with a capacitor in order to form a low frequency inductance-capacitance (LC) band pass filter, effectively shorting unwanted signal frequency components to ground.
Many enclosed systems powered by external alternating current wiring are also shielded from EMI by the incorporation of internal shields. As one example, a metal cabinet housing which encloses the system may be designed to function as a shield. However, metal housings are often too expensive or heavy for portable applications. To avoid some of the weight and expense, the inside of a molded plastic housing may be coated with a thin metal film. Sometimes metal-filled paints are applied to the housing. If cost permits, metal-filled plastic is sometimes used to form the housing. In most cases these different types of shields are connected to ground potential. Any break in the shield will form an aperture through which radiation will emit. Thus, great care is taken to use conductive gaskets to seal access areas. Also within housings, a conductive metal screening may be inserted in the air flow path of a fan-powered cooling system to help reduce radiated emissions from the cooling or exhaust port.
A common feature of these and other prior art EMI shielding methods is that the prior art methods are attempt to shield EMI in a broad sense using an "all-encompassing" shield material. However, such broad approaches are not sufficiently effective. As an example, a metal housing may shield the enclosed electronic devices from various E-field components of EMI, but not H-field components. Accordingly, an alternative shielding method which more specifically targets shielding of particular EMI frequency ranges would be welcome. Particularly, such a method should effectively prevent radiative emissions and protect system components from a selected spectrum of impinging electromagnetic radiation without significant additions to system size, mass, or cost. Fulfilling this need is a growing concern as portable electronics and communications become increasingly popular.